1. Field of the Invention
The present invention relates to a branching circuit that branches and transfers, from a signal line in common to a plurality of communication signals having different frequency bands, the communication signals to signal lines are different for each of the communication signals, and also relates to an RF circuit module including the branching circuit.
2. Description of the Related Art
Hitherto, a branching circuit has been widely used as a circuit element to perform transmission and reception of one communication signal through a single antenna in common. A branching circuit has also been used in the case of receiving reception signals as a plurality of communication signals and transmitting transmission signals as a plurality of communication signals through a single antenna.
For example, when a branching circuit is used to perform transmission and reception of one communication signal through a single antenna in common, a transmission signal input terminal, a reception signal output terminal, and an antenna connection terminal are disposed in the branching circuit. The branching circuit is designed to output the transmission signal, which has been input through the transmission signal input terminal, from the antenna connection terminal without outputting the transmission signal from the reception signal output terminal. Furthermore, the branching circuit is designed to output the reception signal, which has been input through the antenna connection terminal, from the reception signal output terminal without outputting the reception signal from the transmission signal input terminal.
To realize the above-described branching process, the branching circuit includes, for example, a transmission-side filter connected between the antenna connection terminal and the transmission signal input terminal, and a reception-side filter connected between the antenna connection terminal and the reception signal output terminal, as disclosed in Japanese Unexamined Patent Application Publication No. 2007-60411. The transmission-side filter is configured such that a frequency band of the transmission signal falls within a pass band of the transmission-side filter, and such that a frequency band of the reception signal falls within an attenuation band of the transmission-side filter. The reception-side filter is configured such that a frequency band of the reception signal falls within a pass band of the reception-side filter, and such that a frequency band of the transmission signal falls within an attenuation band of the reception-side filter.
At present, an RF circuit module including the above-described branching circuit is constituted by a multilayer body in which conductor patterns for realizing a predetermined circuit are formed, and by circuit elements mounted on the multilayer body.
In the branching circuit, there is a portion where conductor patterns through which the transmission signal is transferred and conductor patterns through which the reception signal is transferred are connected to each other. Hence the transmission signal tends to come into the reception side, and the reception signal tends to come into the transmission side. Usually, as described above, the transmission-side filter attenuates the reception signal, and the reception-side filter attenuates the transmission signal.
With downsizing of the RF circuit module, however, the conductor patterns on the transmission side and the conductor patterns on the reception side are positioned closer to each other and are apt to more easily cause coupling therebetween. In addition, power of the transmission signal is large. Accordingly, a portion of the transmission signal may leak to the reception signal output terminal. This gives rise to, for example, a problem that characteristics of the reception circuit in a downstream stage degrade.